Doc for ccall with `Union{}` return type

[ihnorton]

While trying to repro the LibExpat failure from this PkgEval run, I ran in to the following, with both LLVM 3.3 and 3.7.1:

Version 0.5.0-dev+2137 (2016-01-14 22:40 UTC)
Commit b2b60c4* (0 days old master)
x86_64-apple-darwin15.2.0

julia> xph = ccall((:XML_ParserCreate, :libexpat), Ptr{Void}, (Ptr{Void},), C_NULL)
Ptr{Void} @0x00007fbc2c26edb0

julia> ccall((:XML_ParserFree, :libexpat), Union{}, (Ptr{Void},), xph)

signal (11): Segmentation fault: 11
while loading no file, in expression starting on line 0
[inline] at /Users/inorton/git/julia/src/jltypes.c:69
jl_is_type at /Users/inorton/git/julia/src/jltypes.c:2064
jl_inst_concrete_tupletype_v at /Users/inorton/git/julia/src/jltypes.c:2206
jl_f_tuple at /Users/inorton/git/julia/src/builtins.c:666
eval_user_input at REPL.jl:65
jlcall_eval_user_input_23657 at  (unknown line)
[inline] at REPL.jl:92
anonymous at task.jl:63
[inline] at /Users/inorton/git/julia/src/./julia.h:1391
jl_apply at /Users/inorton/git/julia/src/task.c:246
Segmentation fault: 11

[vtjnash]

that's supposed to segfault. the Union{} return type means "I don't return" (e.g. jl_error or longjmp), not "I return nothing"

[ihnorton]

That's kind of surprising, and it doesn't seem to be documented. Can we do something like ccall( ..., :noreturn, ...), and make the Union{} construct an error?

Changing to a doc issue for now.

[TotalVerb]

I don't think it is so surprising.

julia> isa(nothing, Union{})
false

julia> isa(nothing, Void)
true

Seems consistent to me.